Microelectronic imagers are used in digital cameras, wireless devices with picture capabilities, and many other applications. Cell phones and Personal Digital Assistants (PDAs), for example, are incorporating microelectronic imagers for capturing and sending pictures. The growth rate of microelectronic imagers has been steadily increasing as they become smaller and produce better images with higher pixel counts.
Microelectronic imagers include image sensors that use Charged Coupled Device (CCD) systems, Complementary Metal-Oxide Semiconductor (CMOS) systems, or other solid-state systems. CCD image sensors have been widely used in digital cameras and other applications. CMOS image sensors are also quickly becoming very popular because they are expected to have low production costs, high yields, and small sizes. CMOS image sensors can provide these advantages because they are manufactured using technology and equipment developed for fabricating semiconductor devices. CMOS image sensors, as well as CCD image sensors, are accordingly “packaged” to protect their delicate components and to provide external electrical contacts.
An image sensor includes an array of pixels arranged in a focal plane. Each pixel includes a photogate, photoconductor, or a photodiode with a doped region for accumulating a photo-generated charge. Microlenses are commonly placed over imager pixels. A microlens is used to focus light onto the initial charge accumulation region. Conventional technology uses a single microlens with a polymer coating, which is patterned into squares or circles over corresponding pixels. The microlens is heated during manufacturing to shape and cure the microlens. Use of microlenses significantly improves the photosensitivity of the imaging device by collecting light from a large light-collecting area and focusing the light onto a small photosensitive area of the sensor. The ratio of the overall light-collecting area to the photosensitive area of the sensor is known as the fill factor of the pixel.
The use of smaller-sized microlens arrays is of increasing importance in microlens optics because of the need to reduce the size of imager devices and increase imager resolution. Reducing pixel size, however, reduces the size of the charge accumulation area in the individual pixels. Accordingly, as the size of imager arrays and photosensitive regions of pixels decreases, it becomes increasingly difficult to provide a microlens capable of focusing incident light onto the photosensitive region. This problem is due in part to the difficulty in constructing a smaller microlens that has the optimal focal characteristics for the imager device and that optimally adjusts for optical aberrations introduced as the light passes through the various device layers. Also, it is difficult to correct possible distortions created by multiple regions above the photosensitive area, which result in increased crosstalk between adjacent pixels. “Crosstalk” can occur when off-axis light strikes a microlens at an obtuse angle. The off-axis light passes through planarization regions and a color filter, misses the intended photosensitive region, and instead strikes a photosensitive region in an adjacent pixel. Consequently, imagers with smaller-sized microlenses have difficulty in achieving high color fidelity and signal/noise ratios.